Ruler/protractor reports doubled measure
Scales values aren't dimensions. Scale works as a factor of the object's original dimensions.
I.E if you look at the default cube, its scales are 1:1:1, but if you look at its dimensions: 2:2:2.
If you set the scale at 2, the dimensions will be 4:4:4. If you set the sclaes to 0.5, dimensions will be 1:1:1.